open —
open or create a file for reading or writing
SYNOPSIS
#include <fcntl.h> intopen(const char *path, int flags, mode_t mode);
DESCRIPTION
The file name specified by
path
is opened
for reading and/or writing as specified by the
argument
flags
and the file descriptor returned to the calling process.
The
flags
argument may indicate the file is to be
created if it does not exist (by specifying the
O_CREAT
flag), in which case the file is created with mode
mode
as described in
chmod(2)
and modified by the process' umask value (see
umask(2/)).
The flags specified are formed by
OR
the following values:
O_RDONLY
Open for reading only.
O_WRONLY
Open for writing only.
O_RDWR
Open for reading and writing.
O_NONBLOCK
Do not block on open or for data to become available.
O_APPEND
Append on each write.
O_CREAT
Create file if it does not exist.
O_TRUNC
Truncate size to 0.
O_EXCL
Error if create and file exists.
O_SYNC
Perform synchronous I/O operations.
O_SHLOCK
Atomically obtain a shared lock.
O_EXLOCK
Atomically obtain an exclusive lock.
O_NOFOLLOW
If last path element is a symlink, don't follow it.
Opening a file with
O_APPEND
set causes each write on the file
to be appended to the end.
If
O_TRUNC
and a writing mode are specified and the
file exists, the file is truncated to zero length.
If
O_EXCL
is set with
O_CREAT
and the file already
exists,
open();
returns an error.
This may be used to implement a simple exclusive access locking mechanism.
If either of
O_EXCL
or
O_NOFOLLOW
are set and the last component of the pathname is
a symbolic link,
open();
will fail even if the symbolic
link points to a non-existent name.
If the
O_NONBLOCK
flag is specified, do not wait for the device or file to be ready or
available.
If the
open();
call would result
in the process being blocked for some reason (e.g., waiting for
carrier on a dialup line),
open();
returns immediately.
This flag also has the effect of making all subsequent I/O on the open file
non-blocking.
If the
O_SYNC
flag is set, all I/O operations on the file will be done synchronously.
A FIFO should either be opened with
O_RDONLY
or with
O_WRONLY.
The behavior for opening a FIFO with
O_RDWR
is undefined.
When opening a file, a lock with
flock(2)
semantics can be obtained by setting
O_SHLOCK
for a shared lock, or
O_EXLOCK
for an exclusive lock.
If creating a file with
O_CREAT,
the request for the lock will never fail
(provided that the underlying filesystem supports locking).
If
open();
is successful, the file pointer used to mark the current position within
the file is set to the beginning of the file.
When a new file is created it is given the group of the directory
which contains it.
The new descriptor is set to remain open across
execve(2)
system calls; see
close(2)
and
fcntl(2).
The system imposes a limit on the number of file descriptors
open simultaneously by one process.
getdtablesize(3)
returns the current system limit.
RETURN VALUES
If successful,
open();
returns a non-negative integer, termed a file descriptor.
Otherwise, a value of \-1 is returned and
errno
is set to indicate the error.
ERRORS
The named file is opened unless:
[ENOTDIR]
A component of the path prefix is not a directory.
[ENAMETOOLONG]
A component of a pathname exceeded
{NAME_MAX}
characters, or an entire path name exceeded
{PATH_MAX}
characters.
[ENOENT]
O_CREAT
is not set and the named file does not exist.
[ENOENT]
A component of the path name that must exist does not exist.
[EACCES]
Search permission is denied for a component of the path prefix.
[EACCES]
The required permissions (for reading and/or writing)
are denied for the given flags.
[EACCES]
O_CREAT
is specified,
the file does not exist,
and the directory in which it is to be created
does not permit writing.
[ELOOP]
Too many symbolic links were encountered in translating the pathname,
or the
O_NOFOLLOW
flag was specified and the target is a symbolic link.
[EISDIR]
The named file is a directory, and the arguments specify
it is to be opened for writing.
[EINVAL]
The flags specified for opening the file are not valid.
[EROFS]
The named file resides on a read-only file system,
and the file is to be modified.
[EMFILE]
The process has already reached its limit for open file descriptors.
[ENFILE]
The system file table is full.
[ENXIO]
The named file is a character special or block
special file, and the device associated with this special file
does not exist.
[ENXIO]
The named file is a FIFO, the
O_NONBLOCK
and
O_WRONLY
flags are set, and no process has the file open for reading.
[EINTR]
The
open();
operation was interrupted by a signal.
[EOPNOTSUPP]
O_SHLOCK
or
O_EXLOCK
is specified but the underlying filesystem does not support locking.
[EWOULDBLOCK]
O_NONBLOCK
and one of
O_SHLOCK
or
O_EXLOCK
is specified and the file is already locked.
[ENOSPC]
O_CREAT
is specified,
the file does not exist,
and the directory in which the entry for the new file is being placed
cannot be extended because there is no space left on the file
system containing the directory.
[ENOSPC]
O_CREAT
is specified,
the file does not exist,
and there are no free inodes on the file system on which the
file is being created.
[EDQUOT]
O_CREAT
is specified,
the file does not exist,
and the directory in which the entry for the new file
is being placed cannot be extended because the
user's quota of disk blocks on the file system
containing the directory has been exhausted.
[EDQUOT]
O_CREAT
is specified,
the file does not exist,
and the user's quota of inodes on the file system on
which the file is being created has been exhausted.
[EIO]
An I/O error occurred while making the directory entry or
allocating the inode for
O_CREAT.
[ETXTBSY]
The file is a pure procedure (shared text) file that is being
executed and the
open();
call requests write access.
[EFAULT]
path
points outside the process's allocated address space.
[EEXIST]
O_CREAT
and
O_EXCL
were specified and the file exists.
[EOPNOTSUPP]
An attempt was made to open a socket (not currently implemented).
The
open();
function conforms to
and
POSIX
specifies three different flavors for synchronous I/O:
O_SYNC,
O_DSYNC,
and
O_RSYNC.
In
OpenBSD,
these are all equivalent.
The
O_SHLOCK,
O_EXLOCK,
and
O_NOFOLLOW
flags are non-standard extensions and should not be used if portability
is of concern.
HISTORY
An
open();
function call appeared in
Version 2 AT&T UNIX.
CAVEATS
The
O_TRUNC
flag requires that one of
O_RDWR
or
O_WRONLY
also be specified, else
EINVAL
is returned.